Copper alloys containing 20-30 wt. % lead, also known as highly-leaded bronze, are commonly used due to benefits such as high strength, high ductility, high melting temperature, and high lubricity. Highly-leaded bronze is often used in rotating shaft bearings such as plain journal bearings or sleeve bearings, where the presence of adequate additional lubrication fluid is uncertain or periodically interrupted. The lubricity in highly-leaded bronze is provided by a lead-based second phase which forms during solidification. The lubricity is at least partially proportionate to the volume fraction of this lead-based second phase, which in turn is proportionate to the amount of lead in the alloy.
Due to health and environmental regulations, some of which are pending at the moment, it can be desirable to substantially reduce or eliminate the use of lead in copper alloys. To be called “lead-free,” lead must constitute less than 0.10 wt. % of the alloy. However, lead-free substitutes for highly-leaded bronze have not been forthcoming. As a result, manufacturers frequently request exemptions from regulations for the use of highly-leaded bronze. For example, a leading manufacturer of compressors used in air-conditioning and heat pumps has recently requested to continue the exemption (9b) for “lead in lead-bronze bearing shells and bushes” from the Restriction of Hazardous Substances directive. Thus, there has developed a need for lead-free, high-strength, high-lubricity copper alloys.